Add a sanity test: check if the generated pb files are up-to-date

extensions/include/grpc++/impl/reflection.pb.h

extensions/reflection/reflection.pb.cc

tools/codegen/extensions/gen_reflection_proto.sh

tools/distrib/check_generated_pb_files.sh

tools/dockerfile/grpc_check_generated_pb_files/Dockerfile

...ockerfile/grpc_check_generated_pb_files/check_pb_files.sh

tools/run_tests/sanity/sanity_tests.yaml